Pressure-sensitive adhesives (PSAs) are used for many applications. They generally require a balance of adhesion and cohesiveness to adhere well to various types of substrates, and to remove cleanly after a long period of adhesion. One type of PSAs, hot-melt PSAs (HMPSAs), has gained popularity over the years.
According to the Pressure-Sensitive Tape Council, pressure-sensitive adhesives (PSAs) are known to possess properties including the following: (1) aggressive and permanent tack, (2) adherence with no more than finger pressure, (3) sufficient ability to hold onto an adherend, and (4) sufficient cohesive strength to be removed cleanly from the adherend. Materials that have been found to function well as PSAs include polymers designed and formulated to exhibit the requisite viscoelastic properties resulting in a desired balance of tack, peel adhesion, and shear holding power. PSAs are characterized by being normally tacky at room temperature (e.g., 20° C.). Materials that are merely sticky or adhere to a surface do not constitute a PSA; the term PSA encompasses materials with additional viscoelastic properties.
These requirements for pressure-sensitive adhesives are assessed generally by means of tests which are designed to individually measure tack, adhesion (peel strength), and cohesion (shear holding power), as noted by A. V. Pocius in Adhesion and Adhesives Technology: An Introduction, 2nd Ed., Hanser Gardner Publication, Cincinnati, Ohio, 2002. These measurements taken together constitute the balance of properties often used to characterize a PSA.
Hot-melt pressure-sensitive adhesives (hereinafter HMPSAs) are widely used in industry. HMPSAs are essentially 100 percent solids systems. Usually, such systems have no more than about 5 percent organic solvents or water, more typically no more than about 3 percent organic solvents or water. Among hot-melt adhesive chemistries, (meth)acrylates (i.e., methacrylates and/or acrylates) are one of the most prominent. (Meth)acrylates have evolved as a preferred class of adhesives due to their clarity, permanence of properties over time, and versatility of adhesion, to name just a few of their benefits.
Typically, acrylic HMPSAs are crosslinked in order to provide desirable shear properties, and various techniques have been used. One method of crosslinking acrylic HMPSAs involves using actinic (e.g., electromagnetic or particulate) radiation, typically ultraviolet (UV) light or electron beam. Another method involves chemical crosslinking through covalent and/or ionic bonds